It's a LED light originally conceived as an accesory for cyclists, but it could be used for any purpose. The frequency at which it blinks depends on the resistors and capacitors used. It can be calculated using this calculator for 555 Astable Circuits.

In the end we settled with the following values that set the frequency to 1.215 Hz or roughly 823ms.

DISCLAIMER During this instructable you'll see a 4,7 kohm resistor instead of the 100 kohm we finally settled with. This was a change we did at the last moment and we couldn't reshoot the photos. Just take the 4.7 kohm resistor as if it was a 100 kohm one.